At the top of our head, the zenith, we have the least possible air we can have. Going towards the horizon, any direction (North, South, East or West), the air quantity increases. This gets severe as we approach the horizon, which means: the air has the highest mass towards the horizontal direction than the zenithal direction.
Last night, using Celestron C14 telescope and SBIG CCD camera, i imaged at 11 different altitude angles, starting from the zenith and coming down all the way towards Eastern Horizon. In the above image you can see these altitude angles at which my telescope was pointed (Blue Bars).
Every set (median combined) has a total exposure of 200 seconds. You can see the background values (Orange Bars) increasing towards Eastern Horizon. The unit is called ADU or Analogue to Digital Unit. This simply means that there is more atmosphere which is holding more city light in the sky and that is what makes the horizon brighter than the Zenith Sky.
So the lesson is.. always try to observe or image the sky at our near the zenith. There the atmosphere is as thin as possible and seeing will be as good as it can get.